Autoimmunity is the third most common category of disease in the United States, affecting 8% of the population. Antibody (Ab) repertoire cloning is a state-of-the-art technology for characterizing Abs produced from peripheral B cells. Studies of human Ab repertoires after HIV or influenza infection have identified Abs that neutralize infection and how they develop. This knowledge was gained from analysis of the VH genes used, patterns of somatic mutation, epitope mapping, and pathogenicity assays, and may ultimately lead to more effective vaccination strategies. We anticipate that these same advancements can be made, and are greatly needed to improve the safety of therapy, in autoAb-mediated diseases. Pemphigus vulgaris (PV) is a potentially fatal autoimmune disease in which Abs against the skin cell adhesion protein desmoglein (Dsg) 3 cause severe blistering of the skin and mucous membranes. It is an ideal disease for Ab repertoire cloning because the antigen is well defined and the disease relevance of cloned Abs can be directly tested by functional assays. Currently, it is unknown how many different anti-Dsg3 Abs exist within a patient, whether anti-Dsg3 Abs are similar among patients, and how they develop. This information is essential to understand the pathophysiology of disease and design safer treatment strategies. Human Abs contain two major structural domains, the variable and constant regions, whose expression is governed by VH-DH-JH and CH gene segment usage, respectively. Neutralizing Abs after viral infection often demonstrate VH gene restriction (such as VH1-46 for rotavirus and HIV, and VH1-69 for influenza), presumably because a limited number of Ab gene segments are optimal for binding specific antigenic epitopes. IgG4 CH restriction is observed in conditions of chronic antigen stimulation, such as individuals undergoing allergic desensitization and beekeepers. Our preliminary data suggest that VH1-46 and IgG4 are predominant in the PV anti-Dsg3 Ab repertoire, an exciting finding because it indicates that there are common features of the autoimmune response among patients. We hypothesize that anti-Dsg Abs use a restricted set of variable and constant region genes, which reflects common mechanisms of autoimmunity in PV. To test this hypothesis we will clone anti-Dsg Ab repertoires from PV patients to determine if the anti-Dsg Ab response is oligoclonal, VH- and IgG4-restricted, and shared among patients. We will define early and late molecular events in B cell development (including formation of the na?ve repertoire by VDJ recombination, somatic hypermutation, and class switch recombination) that contribute to the formation of autoreactive Abs in PV. The proposed studies will identify how autoimmunity to a disease-relevant antigen in humans occurs and may lead to novel strategies for targeting only the disease-specific Abs, sparing the beneficial Abs that protect from infection.